A cracked or broken exhaust manifold is most often caused by one of two things. The first reason is heat cycles and/or age. Over time, the manifolds just can't take the heat anymore (literally and figuratively), and cracks begin to form in them. The cracks then turn into small holes, and it doesn't get any better from there. The second common reason for exhaust manifold failure is because of broken exhaust system hangers. Your whole exhaust system is supposed to be held up with a bunch of hangers. If those hangers break off, or the rubber donut exhaust hangers tear apart, all of that weight becomes the responsibility of the exhaust manifold. Sadly, they aren't built for that kind of action. Before long, they give in to the extreme weight hanging off of them, and fail in grand fashion.